- For two hours...
Then silence again...
Still sensitive to touching the switch bank.
- ?
I tend to get quite hooked on solving strange faults, especially stubborn ones
like this, so back on the bench it went.
Every switch contact was tested for continuity with the meter.
Each and every one of them found to be functioning correctly. Also when warm.
- What is going on here ?
That was when my eyes fell on the bottom cover that was put aside while the set was
on the bench.
Full of holes and with its upper side covered with aluminum foil, it provides
shielding under the high-impedance and high-frequency circuits while allowing for
ventilation to the warm components inside the set.
The aluminum foil had losened and lifted in some areas. A couple of places it even had
little imperfections in the form of raised tears and bends.
Hmm... what if this cover bends upwards ever so slightly and/or the main PCB board bends
downwards the same way when warming up, could one or more of these fairly small
imperfections actually reach up and touch a solder joint or a component on the underside
of the main board ?
Certainly !
And it's aluminum, conductive and electrically connected to the chassis when mounted
so would have a lot of potential for causing a fault like this - or even worse.
The tears were smoothed out and the foil was glued down where it had losened from
the 3mm masonite base of the bottom cover.
While working with the glue, the set was testet without the cover to confirm the diagnose.
Confirmed !
It played fine for hours on the bench so the cover was mounted again and the set
transported to the kitchen once more.
Martin
